Hard disk drives incorporate magnetic storage disks and read/write heads that are capable of reading data from and writing data onto the rotating storage disks. Data is typically stored on each magnetic storage disk in a number of concentric tracks on the disk. The read/write heads, also referred to as read/write transducers or read/write elements, are integrated within a slider. The slider, in turn, is part of an actuator assembly which positions the heads relative to the surface of the storage disks. This may be at a predetermined height above the corresponding storage disk or, in some instances, in contact with the surface of the storage disk. The actuator assembly is typically positioned by a voice coil motor which acts to position the slider over a desired track. One or more read/write heads may be integrated within a single slider. In the case of non-contact sliders, a cushion of air is generated between the slider and the rotating disk. The cushion is often referred to as an air bearing.
Hard disk drives are an efficient and cost effective solution for data storage. Depending upon the requirements of the particular application, a disk drive may include anywhere from one to a plurality of hard disks and data may be stored on one or both surfaces of each disk. While hard disk drives are traditionally thought of as a component of a personal computer or as a network server, usage has expanded to include other storage applications such as set top boxes for recording and time shifting of television programs, personal digital assistants, cameras, music players and other consumer electronic devices, each having differing information storage capacity requirements.
A primary goal of disk drive assemblies is to provide maximum recording density on the storage disk. In order to provide greater storage capacity on a storage disk, track widths have become increasingly more narrow. However, decreasing the width of tracks makes it more difficult for the read/write heads to accurately read and write information to and from the narrowing tracks. Not only is it difficult to physically position the read/write element over a narrow width track, but it is increasingly difficult to maintain the read/write element over the track at an optimal position for accurate data transfer. Air turbulence created by the spinning disks, disk flutter and spindle vibrations, temperature and altitude can all adversely effect registration of the read/write element relative to the tracks. Moreover, increasing the speed of the rotating disks to achieve increased data access times increases air turbulence, which increases misregistration between the read/write element and the tracks on the storage disks (track misregistration or TMR). Higher rotational speeds can also increase disk flutter and spindle vibrations further increasing TMR. Higher rotational speeds can also increase spindle motor power and idle acoustics.
Accuracy can be further adversely affected if the read/write heads are not maintained within an optimum height range above the surface of the storage disk. Thus, a related goal is to increase reading efficiency or to reduce reading errors, while increasing recording density. Reducing the distance between the magnetic transducer and the recording medium of the disk generally advances both of those goals. Indeed, from a recording standpoint, the slider is ideally maintained in direct contact with the recording medium (the disk) to position the magnetic transducer as close to the magnetized portion of the disk as possible. Contact positioning of the slider permits tracks to be written more narrowly and reduces errors when writing data to the tracks. However, since the disk rotates many thousands of revolutions per minute or more, continuous direct contact between the slider and the recording medium can cause unacceptable wear on these components. Excessive wear on the recording medium can result in the loss of data, among other things. Excessive wear on the slider can result in contact between the read/write transducer and the disk surface resulting, in turn, in failure of the transducer, which can cause catastrophic failure.
Similarly, the efficiency of reading data from a disk increases as the read element is moved closer to the disk. Because the signal to noise ratio increases with decreasing distance between the magnetic transducer and the disk, moving the read/write element closer to the disk increases reading efficiency. As previously mentioned, the ideal solution would be to place the slider in contact with the disk surface, but there are attendant disadvantages. In non-contact disk drives there are also limitations on how close a read/write element may be to the surface of a disk. A range of spacing is required for several reasons, including the manufacturing tolerances of the components, texturing of the disk surface and environmental conditions, such as altitude and temperature. These factors, as well as air turbulence, disk flutter and spindle vibration, can cause the read/write element flying height to vary or even cause the read/write element to contact the spinning disk.
Disk drives are assembled in a clean room to reduce contamination from entering the drive prior to final assembly. Thus, the air that is trapped within the drive once it is finally sealed is filtered room air. Accordingly, seals used in disk drives between the housing components, such as the base plate and cover, are designed to prevent contaminants from entering the drive. Such seals are not designed to prevent internal air and other gases from exiting through the seal and out of the drive. Loss of gas in this manner is anticipated and accommodated by use of a filtered port to maintain equalized air pressure within the drive compared to that of air pressure outside of the drive.
As an alternative to air-filled drives, advantages may be achieved by filling disk drives with gases having a lower density than air. For example, helium has a lower density than air at similar pressures and temperatures and can enhance drive performance. As used herein, a low density gas or a lower density gas means a gas having a density less than that of air. When compared with air, lower density gases can reduce aerodynamic drag experienced by spinning disks within the drive, thereby reducing power requirements for the spindle motor. A low density gas-filled drive thus uses less power than a comparable disk drive that operates in an air environment. Relatedly, the reduction in drag forces within the low density gas-filled drive reduces the amount of aerodynamic turbulence that is experienced by drive components such as the actuator arms, suspensions and read/write heads. Reduction in turbulence allows drives filled with low density gas to operate at higher speeds compared with air-filled drives, while maintaining the same flying height and thereby maintaining the same range of read/write errors. Low density gas-filled drives also allow for higher storage capacities through higher recording densities due to the fact that there is less turbulence within the drive which allows the tracks to be spaced more closely together.
The die casting process, as well as other methods of manufacturing housing components, often results in the components having a porosity (small pock mark-shaped craters or pits) at the surface and within the body of the component (small voids in the grain structures of the material). This porosity can inhibit or prevent an adequate seal between two abutting surfaces of two different components when there are pits or craters on the abutting surfaces and, similarly, can prevent an adequate seal of openings in a component, such as an opening in a base plate for a spindle motor, when the act of forming the opening exposes air pockets in the body of the component. Additionally, surface porosity can inhibit or prevent adequate sealing between an assembly of two parts that includes an epoxy or adhesive material at the interface. Porosity within the body of the components can also allow low-density gas to permeate through the walls of the enclosure. Porosity of these kinds must be accounted for when making a low-density gas filled disk drive.
To achieve hermetic sealing, some components of the disk drive, usually the die castings, can be treated with a sealant that is intended to reduce the porosity of the components, thereby reducing the amount of gas allowed to escape the disk drive. Most sealant treatment methods typically employ an autoclave or similar vessel for holding die castings and a means for sealant storage within the autoclave or via a discrete tank. In the case where a discrete storage tank is used, the autoclave is closed and sealed and a vacuum is pulled into the autoclave. A transfer valve between the storage tank and autoclave is opened and sealant is forced from the storage tank (at atmospheric pressure) to the autoclave (at vacuum pressure) where the pressurized environment forces the sealant into the surface cavities. The large pressure gradient, sometimes as large as 14.7 psia, between the storage tank and autoclave also causes the sealant to flow at a relatively high velocity resulting in a turbulent flow. As a consequence of the turbulent flow, the sealant begins to foam and encapsulate air.
Once the castings are fully submerged in the sealant, the transfer valve between the autoclave and storage tank is closed. The vacuum pressure in the autoclave is maintained for a predetermined period of time, and then atmospheric air is vented into the autoclave to force the sealant into the evacuated pores and crevasses in the castings. While the sealant is being forced into the pores of the casting, vacuum pressure is created in the storage tank.
After the sealant has been allowed to substantially penetrate pores and crevasses of the castings, the transfer valve between the autoclave and storage tank is opened. At this point there is atmospheric or increased pressure in the autoclave and vacuum pressure in the storage tank. Due to this pressure difference between the autoclave and storage tank, sealant is moved back to the storage tank at a relatively high velocity again resulting in foaming of the sealant due to turbulent flow. When the sealant is returned to the storage tank, the transfer valve is closed again and atmospheric air can be reintroduced to the storage tank. The autoclave is then opened and the castings are removed. The autoclave then waits in stand-by mode until another impregnation cycle is desired.
A problem with the impregnation cycle described above is the turbulent flow of sealant between the autoclave and storage tank causes the sealant to cavitate and create gas bubbles. When the sealant is impregnated into the castings, the gas bubbles may also be trapped therein. If the gas bubbles burst during the pressurized sealing process additional sealant will fill the void left by the burst bubble. However, if a gas bubble subsequently remains in a casting pore or crevasse, after the casting has been removed from the autoclave, an unsealed surface void remains which may ultimately lead to leakage of gas from the disk drive. The warranted life of an average disk drive may be decreased significantly if too much low density gas is allowed to exit the disk drive. As the life of the disk drive decreases, so does the potential market value of the disk drive.
In addition, the existence of dissolved or suspended air in the sealant further impedes the ability of the sealant to penetrate pores and crevasses of the casting. This condition can lead to variations in sealant penetration uniformity over the casting. The addition of bubbles to the sealant also decreases the permeability of the sealant. Thus, under turbulent flow conditions, the permeability of the sealant degrades as it is moved between the storage tank and autoclave, ultimately resulting in a lower quality sealant. Of course, the bubbles can be removed from the sealant, but this “de-gassing” process is time consuming and decreases the efficiency of the overall disk drive manufacturing process.
Another problem with the impregnation cycle of the prior art is that debris and other particulate matter may be introduced to the sealant as it passes between and sits in the autoclave and storage tank. The sealant is initially bought as a relatively “clean” product, meaning that it has few impurities. However, as the sealant is reused and moved between the autoclave and storage tank, the cleanliness of the sealant may become compromised. Any particles or debris that are carried to the autoclave with the castings or while the autoclave is open, or are otherwise introduced to the sealant during the impregnation process will likely remain in the sealant. It is important to maintain a clean sealant because if the impurities are trapped in a pore or crevasse of the casting and later become dislodged a passageway for gas to exit the disk drive may be created.